A. Technical Field
This invention relates to data storage and retrieval, and more particularly to the operation of a SATA drive that interfaces within a SAS domain.
B. Background of the Invention
The growth and importance of data storage technology are well known. This technology growth includes the development of different storage-related protocols and standards which define structures and methods by which data may be stored and retrieved. As a result of these changes, it is not uncommon for storage systems to have components that operate according to different standards and/or designs. In particular, as a storage system is built-out over time, new products and technologies may come to market that provide performance improvements and may be integrated within legacy systems. Conversely, legacy components and storage devices may be integrated within a storage system in order to keep costs down.
Storage devices may access, store and retrieve data using various techniques. The performance of a storage system may significantly depend on the manner in which its storage devices read and write data to and from disks. The reliability and speed of storage drives, and the corresponding interfaces between these devices thus become important elements of a storage system. Continual improvements in storage technology and standards have drastically improved the performance of serial communication, which has led to the integration of different serial-based interface components and storage drives being integrated within storage systems. Two such types of serially-based interface components and storage drives are Serial Advanced Attachment (“SATA”) and Serial Attached SCSI (“SAS”) devices.
SAS technology defines a point-to-point serial interface capable of full duplex communication at greater dedicated bandwidth than earlier interfaces, such as SATA. In order to aid the integration of SAS within the storage market, SAS technology was defined as being compatible with SATA. This compatibility allows the large number of pre-existing SATA storage devices to be integrated within SAS domains. In particular, the SAS topology allowed SATA-compliant interfaces to be connected within a SAS system domain. As a result, a user is able to install comparatively lower cost and poorer performing SATA drives and higher performing and more expensive SAS drives within a single SAS domain.
FIG. 1 shows an exemplary SAS domain 102 with a SATA storage drive 108 and SAS storage drive 110. A connection between the SATA drive 108 to the SAS domain 102 is made through expanders 104 and 106. An STP Initiator 102b is located within the SAS domain 102 that provides an interface for data from the SATA drive 108. The STP Initiator 102b operates according to the Serial ATA Tunneling protocol (“STP”). Comparatively, a SSP Initiator 102a receives data from the SAS drive 110 and operates according to the Serial SCSI protocol (“SSP”).
The SAS Initiator 102b and the SSP Initiator 102a are connected to a first expander 104, which is connected to a second expander 106 that communicates with the SATA drive 108 and the SAS drive 110. The expanders 104, 106 improve the scalability of the Serial attached SCSI systems by allowing multiple end devices, such as the drives, to a plurality of Initiator devices.
The integration of SATA drives within an SAS domain creates data transfer inefficiencies caused by the comparably different effective data transfer rates between SATA and SAS. In particular, a delay caused by the SATA drives is attributed inherently to the way the SATA drives are designed and the flow control issued by SATA drives. This delay may cause data bottlenecks on intermediate links of the SAS topology because data paths may become congested with the lower rate SATA data. As a result, the data transfer between a SAS disk drive and the SSP Initiator 102a may be significantly delayed, reducing performance, due to the usage of intermediate links by the inefficient SATA drive.
There is a current need for a practical and cost effective solution that addresses congestion caused by SATA drives that are attached to a SAS domain.